(a) Field of the Invention
The present invention relates to a process for stably treating sludge which is generated from a ground improving working site and a system for the same.
(b) Description of the Related Art
Sludge such as cement or bentonite sludge which is discharged from a ground improvement working site contains much cement. Accordingly, it is necessary to treat the sludge as industrial waste. In a construction site where wide yards can be provided, a pit which will contain the sludge is bored and the sludge is sun-dried and is transported. In practice, it is difficult to provide a wide drying yard. Most of the generated sludge (more than 90%) is transported to a final treating site by a vacuum car or sludge transporting dump car without any treatment and is processed, for example, by sun drying by specialists.
On the other hand, in usual dam sites or slurry shield boring sites, the sludge treatment is conducted by preliminarily removing suspended materials having large grain sizes and high specific weights from the slurry in a sand sedimentation pond or by means of liquid cyclone or mud screen and then by flocculating finer particles which have passed the screen with a flocculant. The flocculated and sedimented sludge contains much water and thus can not be transported by a dump car or can not be buried for disposal. Accordingly, the sludge is dehydrated by sun-drying or by means of dehydrating machine and thereafter transported for disposal.
If supernatant water in which suspended material is flocculated and sedimented has a ph which falls with a range of regulation (usually less than 5.8 and higher than 8.6), it is pH adjusted with a neutralizing agent and then discharged or reused.
The dehydrating treatment may include vacuum dehydration by means of belt filter, oliver filter and the like; centrifugal dehydration by means of centrifugal dehydrating machine; pressurizing dehydration by means of filter press, belt press and the like; granulation dehydration by means of granulation dehydrating machine. Many filter presses having a high dehydrating capacity are used because the size of the machine is small and is easy to handle and to transport.
It has become more difficult to provide disposal sites for industrial waste, since they have to be provided in more remote area's in order to avoid pollution problems. The sludge generated from a ground improvement working should be reduced in volume by flocculation and dehydration in order to reduce cost. The sludge generated by jetting and agitating, such as the so called C-JG (column jet) method, JSG method, etc. among various ground improvement working methods, contains much cement or bentonite, and has a relatively high specific weight due to mixing with fine soil particles in association with jetting and agitation, such as a specific weight of about 1.35 to 1.50 as shown in FIGS. 14 and 15. (FIGS. 14 and 15 show the specific weight of cohesive and sand soils respectively) and may have .gamma.=1.60 for specific soil species, such sludge exhibits a number of problems in sludge treatment.
Disposal of the sludge is regulated so that the disposed sludge has a water content of 85% or less. If dehydrated cakes have a water content of about 50% or more, on transportation by dump cars, the cakes readily become slurry for an extended period of transportation.
Unless the dehydrated cakes have a water content less than the above mentioned value, transportation is difficult. A stable dehydrating capacity is one of the most essential requirements for the sludge treatment.
However, even if the cement or bentonite sludge which is generated from a ground improvement working site is continuously treated by adding a flocculant and by dehydrating it by means of a dehydrating machine, clogging of filter fabric which is used in a filter press or belt press may occur due to the fact that the untreated sludge has relatively high varying specific weight and that an insufficient or excessive amount of flocculant may be added. This invites lowering the treating capacity.
Conventional sludge treatment is carried out by means of a sludge treating machine installed in the site and manually operated. Operation and maintenance cost is very high. Automation of the treatment and reduction in the operating cost by reduction in volume of sludge are desired.